We showed previously that E. coli glutamine synthetase (GS) is inactivated in vitro by several mixed function oxidation systems. The inactivation requires O2, is stimulated by Fe+3 and is inhibited by catalase. Nitroghen starvation provokes a similar inactivation of GS in E. coli in situ. In order to test the generality of this phenomenon, we studied the capacity of a microbial NADH-diaphorase system and a rabbit liver microsomal P450 system to inactivate enzymes other than GS. Of the nineteen additional enzymes tested, the diaphorase system inactivated alcohol dehydrogenase, aspartokinase III, creatine kinase, glyceraldehyde-3-phosphate dehydrogenase, lactate dehydrogenase, phosphoglycerate kinase, and pyruvate kinase. Except for aspartokinase III, not yet tested, the same enzymes were inactivated by the P450 system. Participation of hydrogen peroxide as an intermediate in these inactivation reaction is inferred by catalase inhibition. FE+3 is reduced to FE+2 by diaphorase and cytochrome P450 systems. In addition to in vitro inactivation studies, experiments were undertaken in E. coli K12 cells in order to study the possible role of this inactivation in vivo. Techniques used in these studies have included polyacrylamide gel electrophoresis, high pressure liquid chromatography, amino acid analysis, chromatographic techniques and enzymatic assay of functional proteins.